Valve actuators find wide application in a number of industries, such as, power generation of all types, petroleum and petrochemicals, textiles, paper, and food processing. The operating speed and torque provided by a valve actuator are important parameters. Generally, a fixed speed motor is coupled to a gear set to provide just the right combination of speed and torque to a valve. The exact motor and gear set have to be coupled by a manufacturer. If a manufacturer wants to have a quick turnaround time to customer orders, the manufacturer must stock a wide variety of motors and gear sets. Additionally, a specialized workforce must be employed that is knowledgeable in the assembly of sometimes hundreds of different variations on a single generic valve actuator design.
Furthermore, even if multiple customers want a valve actuator with exactly the same speed and torque characteristics, often customers will have different power supplies available. One customer may want to use 480 VAC three-phase at 60 Hz; another customer may want to use 110 VAC single-phase at 50 Hz; and yet another customer may only have 24 VDC available. Previously known systems do not provide a valve actuator that is supplied off-the-shelf to meet the needs of customers with different power supplies available.
Currently, if a user wants to change the torque and speed characteristics of a valve actuator, the actuator has to be pulled from service, disassembled, and then reassembled with a different gear sets and/or motor. What is needed is a way to reduce the number of motors and gear sets that must be stocked to meet customer needs. There is a further need to reduce the number of models that employees must be trained to build. Additionally, there is a need to permit adjustment of the speed and torque delivered by an actuator without requiring disassembly.
Additionally, fixed speed valve actuators have limited utility as process controllers because the valve is always operated at a fixed speed. What is needed is a way to allow a valve actuator to operate as a process controller.
One attempt to solve these problems was through the use of a rectifier and chopper to control the current sent a DC motor. This allowed for high or low voltage AC current that was either single- or three-phase to be used and allowed the speed and torque of the motor to be controlled. A variation on this attempt was to rectify AC, then use an inverter to control an AC motor. However, these attempts required the use of torque limit switches. An operator could mechanically adjust the speed and torque delivered by a valve actuator, but at most, an operator could only set a maximum torque or speed that should not be exceeded by the valve actuator. An operator could not set a speed or torque profile that would vary over the length of a valve stroke without limit switches. A gear set was included with the valve actuator and was located within the housing of the valve actuator.
The previous attempts require a user to adjust speed and torque potentiometer and do not permit an operator to set a speed or torque profile that would vary over the length of a valve stroke. These attempts do not provide a way to set the speed and torque of a valve actuator without bulky mechanical switches and torque limit switches, nor do they provide a mechanism where the valve actuator could operate as a process controller.
Other attempts to solve the above problems have used switched reluctance motors and DC motors to provide variable speeds. However, in those cases, the valve actuator requires a separate braking mechanism to keep the motor from spinning in the event of a power loss. What is needed is a valve actuator capable of variable speeds that simply and inherently has braking capabilities.